You could try FunctionExpand, possibly with optional 2nd argument putting assumptions on x. However, your purported result in the first example has a syntax error -- unbalanced parentheses -- which I'm going to assume should actually be:

(1/2 Sqrt[Pi]) x^3 Exp[-x/2] (BesselK[0, x/2] + BesselK[1, x/2])

And I don't think that's the same thing as your

MeijerG[{{}, {5/2}}, {{2, 3}, {}}, x]

(which is correct Mathematica syntax for a MeijerG that, if I understand what you wrote, is what you want). In fact, plotting the two functions reveals the difference.

On Aug 23, 2013, at 5:30 AM, amzoti <amzoti@gmail.com> wrote:

> Does Mathematica have a similar function to this?>> convert(MeijerG([[],[5/2]],[[2,3],[]],x),StandardFunctions);>> which results in: (1/(2 Sqrt[Pi]) x^3 Exp[-x/2] (BesselK[0,x/2]+BesselK[1,x/2]).>> or>> convert(ln(1+x),MeijerG,include=elementary);>> You can see what the result would be here: http://en.wikipedia.org/wiki/Meijer_G-function>> I found this nice list of special functions on the Mathematica web site (http://functions.wolfram.com/HypergeometricFunctions/MeijerG/03/01/03/23/) and can look it up, but would rather be able to go in each direction by typing a command.>> Is there a way to do this in Mathematica?